Soy protein products are used as food additives for enhancing texture and other functional characteristics of various food products as well as a source of protein. The use of soy protein products may be limited in some instances, however, due to their beany flavor and tan-like color. It is still unclear exactly which components are responsible for the flavor and color characteristics of soybeans, though a variety of compounds are suspected of causing these characteristics. Among these are aliphatic carbonyls, phenolics, volatile fatty acids and amines, esters and alcohols.
Soybean protein in its native state is unpalatable and has impaired nutritional quality due to the presence of phytic acid complexes which interfere with mammalian mineral absorption, and the presence of antinutritional factors which interfere with protein digestion in mammals. There are extensive reports of processes used for the isolation, purification and improvement of the nutritional quality and flavor of soybean protein. The reported methods include the destruction of the trypsin inhibitors by heat treatment as well as methods for the removal of phytic acid. A wide variety of attempts to improve the yield of protein secured as purified isolate relative to that contained in the soybean raw material have also been described.
Many processes for improving soy protein flavor involve the application of heat, toasting, alcohol extraction and/or enzyme modification. These types of processes often result in substantial protein denaturation and modification, thereby substantially altering the product""s functionality. In addition, these processes can promote interactions between proteins with lipid and carbohydrate constituents and their decomposition products. These types of reactions can reduce the utility of soy proteins in food products, especially in those that require highly soluble and functional proteins, as in dairy foods and beverages.
Commercial soy protein concentrates, which are defined as soy protein products having at least 70% by weight protein (dry solids basis), are generally produced by removing soluble sugars, ash and some minor constituents. The sugars are commonly removed by extracting with: (1) aqueous alcohol; (2) dilute aqueous acid; or (3) water, after first insolubilizing the protein with moist heating. These processes generally produce soy protein products with a distinctive taste and color.
Soy protein isolates are defined as products having at least 90% by weight protein (dry solids basis). Commercial processes for producing soy protein isolates are generally based on acid precipitation of protein. These methods of production typically include (1) extracting the protein from soy flakes with water at an alkaline pH and removing solids from the liquid extract; (2) subjecting the liquid extract to isoelectric precipitation by adjusting the pH of the liquid extract to the point of minimum protein solubility to obtain the maximum amount of protein precipitate; and (3) separating precipitated protein curd from by-product liquid whey. This type of process, however, still tends to produce a protein product with a distinctive taste and color.
A number of examples of processes for producing concentrated soy protein products using membrane filtration technology have been reported. Due to a number of factors including cost, efficiency and/or product characteristics, however, membrane-based purification approaches have never experienced widespread adoption as commercial processes.
One early method for obtaining soy protein relied on homogenization to obtain a fine dispersion which was subjected to centrifugal separation. The liquid extract obtained from this separation was then subjected to reverse osmosis to remove water and low molecular weight compounds. The retentate from the reverse osmosis was dried to produce the final product.
A number of processes which make use of ultrafiltration in producing soy or other protein products have also been reported. A typical process of this type involves extensive membrane filtration of a relatively dilute oilseed protein extract, followed by concentration and drying of the retentate. The nominal molecular weight cut-off of the ultrafiltration membranes employed in such processes is typically reported to be in the range of 10,000 to 100,000, with membranes having an MWCO in the lower half of this range generally described being preferred. For example, one process described for preparing a soy protein with reduced phytic acid content includes aqueous extraction of defatted soy flakes, basification of the extract to a pH in excess of 10.1 and the removal of resulting insolubles. The liquid phase is then neutralized and subjected to ultrafiltration to retain higher molecular weight protein while allowing the lower molecular weight compounds to pass, e.g., using a semipermeable membrane having a minimum molecular weight in the range of about 10,000-50,000 daltons. A number of other methods which involve membrane filtration of acidic or basic soy protein extracts have been described.
These and other related processes can suffer from one or more disadvantages, such as reduced functional characteristics in the resulting protein product and the production of a product which has an xe2x80x9coffxe2x80x9d flavor and/or an off-color such as a dark cream to light tan color. Membrane-based processes can also be difficult to operate under commercial production conditions due to problems associated with bacterial contamination of the membranes. Contamination can have undesirable consequences for the flavor of the product. In addition, deterioration of the membrane under repeated use can necessitate frequent replacement of membrane modules, greatly increasing process down time and raising process costs.
A method of converting oilseed material, such as defatted soybean white flakes or soybean meal, into a high protein content material with desirable flavor and/or color characteristics is described herein. The modified oilseed material provided by the method is particularly suitable for use as a protein source for incorporation into foods for human and/or animal consumption.
The present process typically includes an extraction step to solubilize proteinaceous material present in an oilseed material. It may be desirable to conduct the extraction as a continuous, multi-stage countercurrent extraction step. If desired, a relatively low water-to-flake ratio can be employed in the extraction process, e.g., no more than about 10:1. This can help reduce the overall hydraulic loading required by the process and produce an oilseed extract that is more concentrated (i.e., has a higher dissolved solids content) than extracts typically produced in conventional processes used to produce oilseed protein concentrates.
The process uses one or more microporous membranes to separate and concentrate protein from the extract. It is generally advantageous to use a microporous membrane which has a filter surface with a relatively low contact angle, e.g., no more than about 40 degrees. The process commonly utilizes either relatively large pore ultrafiltration membranes (e.g., membranes with a molecular weight cut-off (xe2x80x9cMWCOxe2x80x9d) of about 25,000 to 500,000) or microfiltration membranes with pore sizes up to about 1.5xcexc. When microfiltration membranes are employed, those with pore sizes of no more than about 1.0xcexc and, more desirably, no more than about 0.5xcexc are particularly suitable. Herein, the term xe2x80x9cmicroporous membranexe2x80x9d is used to refer to ultrafiltration membranes and microfiltration membranes collectively. By employing such relatively large pore microporous membranes, the membrane filtration operation in the present process can be carried out using transmembrane pressures of less than about 100 psig, desirably less than about 50 psig, and more commonly in the range of 10-20 psig.
In most instances, the permeate stream produced by the membrane filtration has essentially no suspended solids. This allows the soluble carbohydrates in the permeate stream to be separated from the water in this stream via reverse osmosis, if desired. The water passing through the reverse osmosis membrane(s) (xe2x80x9cRO permeatexe2x80x9d) can be recycled back into an earlier stage in the process, e.g., in the extraction or for use as a diluent in a diafiltration operation.
The retentate stream produced by the membrane filtration typically has a relatively high protein content (on a dissolved solids basisxe2x80x94xe2x80x9cdsbxe2x80x9d). For example, the present process can be used to produce oilseed protein products which have at least about 70 wt. % protein (dsb) and, preferably, at least about 90 wt. % protein (dsb). The concentrated soy protein material can be used directly in the form produced by the membrane filtration or may be further processed. For example, the retentate stream is commonly dried to produce a dry particle product, e.g., by spray drying the retentate stream using conventional equipment. When this is done the pH of the retentate stream is typically adjusted to a pH close to neutral (e.g., about 6.8-7.0), prior to spray drying. Depending on the type of product desired, the pH adjusted retentate may also be subjected to treatment at a relatively high temperature for a short time (xe2x80x9cHTSTxe2x80x9d) in order to pasteurize the retentate. Since the HTST step is very effective in killing bacteria, even thermophiles, this operation can be used to substantially reduce the total plate counts of bacteria in the final soy protein product. Depending on the temperature and duration of the treatment, it can also be utilized to alter the functional properties of the soy protein product.